The field of the manufacture of impact modified plastics is relatively old and the current industrial processes for their manufacture are fairly well known. According to conventional technology typically a solution of rubber, typically comprising 1 to about 20, preferably from 3 to 12 weight %, most preferably 4 to 10 weight % of rubber dissolved in one or more monomers is polymerized in a first stage reactor under mechanical agitation. Whether the polymerization occurs in a batch, stirred plug flow or continuous stirred tank reactors, almost all prior art and disclosures clearly teach that the particle size, particle size distribution and morphology of the dispersed rubber-like composite phase of the final product is largely determined during particulation in the early part of the process.
Particulation is the generic term used to describe the formation of the dispersed rubber-like composite phase regardless of its mechanism.
In the production of high impact polystyrene in a batch process or in a stirred plug flow reactor, the rubber-like composite phase is the continuous phase and the resin phase (monomer/resulting polymer phase) is dispersed. Typically, in conventional processes, as the polymerization proceeds in time with a batch reactor or in space with a stirred plug flow reactor at some point between 5 and 20% conversion the system undergoes particulation by phase inversion under the application of a shear field generated by mechanical agitation. That is the rubber-like composite phase becomes the dispersed phase and the resin phase becomes the continuous phase. This does not happen instantaneously but occurs over a considerable period of time or space, typically from 20 to 50 minutes or reactor space which produces 2 to 8% conversion. That is the rubber-like composite phase and resin phase become co-continuous for a period of time or space before the particulation process is complete.
The ternary phase diagram of the styrene-polystyrene-polybutadiene system has been well studied and is well known. For example, the phase diagram and what happens during the polymerization of high impact polystyrene is discussed in Kirk-Othmer Encyclopedia of Chemical Technology published in 1983, Volume 21, pages 823 through 826.
In the production of high impact polystyrene in a continuous stirred tank reactor (CSTR) the rubber phase is particulated by the mechanism of dispersion. That is the rubber or rubber-like composite phase is dispersed in a CSTR that is operated with a continuous resin phase.
The distinction between rubber phase and rubber-like composite phase used in this document is as follows: The rubber phase is simply rubber dissolved in one or more monomers, while the rubber-like composite phase refers to rubber that has been modified by reaction with one or more monomers during polymerization. That is during polymerization polymer chains containing one or more monomers is grafted to the rubber molecules. In addition to graft copolymer, the rubber-like composite phase may contain occluded polymer. Occluded polymer is not grafted to the rubber molecules and resides within the rubber-like composite phase.
According to conventional wisdom the polymer chemist has a limited degree of freedom concerning the process of particulation in the manufacture of impact modified thermoplastic resins. That is particulation is limited to the region of phase inversion in a batch process and stirred plug flow reactors or at the point of dispersion in CSTR's. It is impossible to precisely control particulation in batch or plug flow reactors since it occurs over a period of time or a region of reactor space. In a CSTR particulation by dispersion occurs almost instantaneously, but due to the dynamics of the system the time the particles spend in the reactor is described by an exponential distribution. That is some particles exit the reactor shortly after forming while others may reside much longer. Furthermore, in a CSTR it is difficult, if not impossible to ensure that each unit volume of the reactants under goes the same or comparable shear history. As a result the particle size distribution of the dispersed rubber-like composite phase is typically broadest when formed in a CSTR.
Particle size, particle size distribution and morphology contribute to a number of properties of the product including impact resistance, gloss and translucency. Unfortunately, generally to maximize one property tends to reduce one or more of the other properties of the final polymer. There have been some attempts to overcome these deficiencies by blending resins having different particle sizes. Such an approach is expensive as it requires passing a melt blend of the resins through an extruder. Additionally, the properties of a blend may be lower than that expected from the weighted numerical average of the properties of each of the components in the blend.
The following is representative of the state of the art in the polymerization of impact modified thermoplastics. Almost all techniques largely determine the final particle size of the rubber-like composite phase at the point of phase inversion or dispersion.
U.S. Pat. No. 2,694,692 issued Nov. 16, 1954, assigned to The Dow Chemical Company discloses the desirability and criticality of agitation during the early stages of polymerization of impact modified thermoplastic polymers.
U.S. Pat. No. 3,658,946 issued Apr. 25, 1972, assigned to Badische Aniline-& Soda-Fabrik Aktiengesellschaft (BASF) discloses particle size and distribution of impact modified thermoplastics may be controlled by varying the stirrer speed or shear during the early part of the reaction.
U.S. Pat. No. 3,660,535 issued May 2, 1972 assigned to the Dow Chemical Company discloses stirring or mechanical agitation during the initial stages of polymerization to create the required particle size distribution in the polymerization of an impact modified thermoplastic.
U.S. Pat. No. 3,903,202 issued Sep. 2, 1975 assigned to Monsanto Company teaches dispersing under mechanical agitation a monomer syrup containing rubber into a partially polymerized monomer, during the early stages of polymerization to create the required dispersion of impact modifier throughout the resin phase.
U.S. Pat. Nos. 4,857,587 and 4,861,827 issued Aug. 15 and 29, 1989 respectively, assigned to Fina Technology Inc. discloses the use of mechanical agitation during the early stages of the polymerization of an impact modified thermoplastic to create the required dispersion of rubber throughout the continuous resin phase.
There are three patents which Applicants are aware of which state the control of shear is important in the process.
Canadian Patent 832,523 issued Jan. 20, 1970 to Shell Internationale Research Maatschappij N.V., teaches HIPS containing a bimodal particle size distribution. The HIPS comprises from 70 to 99 weight % of polystyrene and from 1 to 30 weight % of a dispersed rubber phase having a particle size distribution so that from 70 to 97% of the particles have a diameter from 1 to 3 microns and from 30 to 3% of the particles have a diameter from 5 to 25 microns.
The Shell patent teaches controlling agitation or shear during the early stages of polymerization to obtain the required particle distribution. The Shell patent teaches using the shear of a conventional process.
It is interesting to note that while the Shell patent also clearly contemplates blending impact modified polystyrenes (page 4, lines 10-15) and interpolymerizing styrene monomer containing two distinct types of rubber to obtain the required particle size distribution, it does not teach or disclose blending syrups having different particle size distributions and completing the polymerization to directly yield a product having a bi-modal particle size distribution.
U.S. Pat. No. 4,007,234, assigned to Hoechst A. G., issued Feb. 8, 1977 discloses a process for controlling the particle size distribution in high impact styrene copolymers modified with ethylene-propylene rubbers. The polymer is prepared using a mass/mass or mass/suspension process with high shear in the prepolymerizer. The resulting polymer is then subjected to a two stage shearing action. A catalyst is introduced into the polymer prior to or during the second shearing to crosslink the rubber particles and to maintain particle size. While the Hoechst patent teaches shearing the polymer, it does not disclose shearing the syrup as required in the present invention. Additionally, the rubber used in the Hoechst process is EPDM which is not used in the present invention.
U.S. Pat. No. 5,210,132 assigned to the Mitsui Toatsu Chemicals, Inc. issued May 11, 1993 discloses a process which forms a dispersed rubber-like composite phase in a continuous resin phase. The particulated syrup is then subjected to shear in a device having at least three shearing blades or rotors. The shearing rotors and stators are coaxial and have comb like cuts at interposing ends or sections to form a multilayer structure. The result is that the Mitsui patent teaches a multi-zone shear field having at least three different shear rates. The shear field in the device of the Mitsui patent will vary radially outward from the centre of the cylindrical comb like rotor as a function of the distance from the centre of the rotor. The present invention is a process in which at least a portion of the syrup is subjected to a relatively uniform controlled shear field. Accordingly the essential features of the Mitsui patent teaches away from the broadest aspects of the present invention requiring the use of a relatively uniform shear field.
In addition to teaching away from the broadest aspects of the present invention the Mitsui patent totally fails to disclose several other aspects of the present invention. Particularly, the use of relatively uniform shear fields to treat portions of the initial syrup and then recombining the treated syrup to form a bi- or polymodal particle size distribution is not disclosed by the Mitsui patent.
None of the above art suggests a process in which large particle size rubber-like composite phase is produced during the initial stages of polymerization which particles are subsequently reduced in size using a controlled relatively uniform shear field. The shear field of the present invention is relatively uniform in that it is relatively free of shear pulsations (compared to more typical agitators in chemical processes such as turbines, anchors, propellers, screws, or helices). An example of a relatively uniform shear field is that generated between coaxial cylinders in which one or both of the cylinders rotate.
There are a number of advantages of the process of the present invention. The invention permits a high degree of control over the particle size, narrowing of the particle size distribution and the process is easily modified by stream splitting and the application of multiple uniform shear field generating devices to produce bi- or multimodal particle size distributions all of which will lead to a better or better balance of properties. In addition by largely setting the particle size of the dispersed rubber-like composite phase in the later part of the process the transition time between grade changes is reduced, which minimizes the amount of off-grade material that is formed in continuous processes.
Accordingly, the present invention seeks to provide an industrial process to reduce the particle size (e.g. the volume average diameter) of at least a portion of the particles within a syrup comprising a resin phase and a dispersed rubber-like composite phase.
As used in this specification the following terms have the following meanings:
"Resin phase" means a solution of polymer resin dissolved in one or more monomers or the polymer itself;
"Rubber phase" means an uncrosslinked rubber dissolved in one or more monomers, or the rubber itself;
"Rubber-like composite phase" means a composite of a rubber phase as defined above and one or more resin phases as defined above. Said composite may contain resin polymers occluded by or grafted onto the rubber polymers;
"Dispersion" means a system of two or more phases in which one phase forms a continuous phase and the other phases are dispersed as small droplets or particles through the continuous phase;
"Dispersed rubber-like composite phase" means a rubber-like composite phase dispersed throughout a continuous resin phase;
"Particulation" a term used to describe the formation of a dispersed rubber-like composite phase regardless of its mechanism;
"Inverting" or "inversion" or "phase inversion" or "particulation by inversion" means the conversion of a continuous rubber-like composite phase into a dispersed rubber-like composite phase in a continuous resin phase;
"Dispersing" or "phase dispersion" or "particulation by dispersion" means the formation of a dispersed rubber-like composite phase in a continuous resin phase by dispersing with mechanical agitation a rubber phase or continuous rubber-like composite phase into a tank which has a continuous resin phase. Typically, this process occurs in a continuous stirred tank reactor (CSTR); and
"Low mechanical agitation" means an agitation rate which is not sufficient to significantly reduce the particle size within a syrup once particles have been formed. Low mechanical agitation occurs using anchor or turbine or other types of agitators at low rates of rotation. Typically with driven agitators the rates of rotations are less than 15, preferably less than 10 RPM's most preferably as low as possible. Of course one skilled in the art will be aware that the degree of agitation will depend on reactor configuration and appropriate speeds can be determined by routine experimentation after reading this specification.